The present invention relates to a method for producing high resolution resist structures with steep edges.
The production of resist structures plays an important role in microelectronics. For example photoresists are structured using photolithographic means in the manufacture of semiconductor components. One consequence of the recent advances in microelectronics, however, is an increase in the level of integration. Also, with smaller and smaller structures, increasing demands are placed on structure production. To meet these demands, greater resolution or higher contrast is required of the applied photoresists.
When photoresists are structured using photolithographic means, in addition to the technology- and resist-specific parameters, the properties of the stepper or the stepper lens used for irradiation also determine the minimum attainable structure size, critical dimension (CD), as well as the depth of focus (DOF). The stepper-specific variables, exposure wavelength .lambda. and lens numerical aperture (NA), are related to CD and the DOF as follows: EQU CD=f.sub.1 (.lambda./NA) and DOF=.+-.f.sub.2 (.lambda./NA.sup.2);
wherein f.sub.1 and f.sub.2 are factors which are specific to the processing equipment.
For some time, the demands of photolithography have been satisfied by liquid-developable, single-layer resists, particularly those consisting of novolak resins (as a base polymer) and quinone diazides (as a photoactive component). Such resist systems, however, may be unable to meet future requirements. For example, when thick resists are processed using excimer laser steppers in the deep-ultra-violet (DUV) range, it may be impossible to produce relief structures having steep edges and dimensions of less than 0.5 .mu.m. This is particularly true of graduated substrate topographies and highly reflective sub-layers. To produce very small structures, shorter exposure wavelengths and high numerical apertures are needed. Unfortunately, this reduces the range of the depth of focus, making it very difficult to use liquid-developable single-layer resists (with relatively thick resist layers and unavoidable fluctuations in layer thickness) when high resolution on graduated topographies is required. In addition, these systems are not suited for application as DUV resists, particularly due to the high self-absorption of novolak, for example, at 248 nm.
To eliminate the problems associated with the application of liquid-developable single-layer resists, so-called two-layer systems were developed. These systems, however, are more complicated. In the two-layer systems, a single thin upper layer is irradiated and structured (i.e., developed or chemically modified through treatment with an agent). The structure produced in the upper layer serves as a contact mask and is subsequently transferred to the lower layer(s). To structure the lower layer, UV-light provided by a flood exposure (i.e., irradiation without an overlay mask) can be used in conjunction with liquid-development or with dry-development techniques such as reactive ion etching in an oxygen plasma (O.sub.2 /RIE).
Dry-developable single-layer systems have the advantages of the two-layer systems, while being less complex. In these systems, a latent image is produced through the irradiation of the surface of a resist layer that has been deposited on the substrate. The resist is then treated with a metal-containing organic reagent (e.g., an organosilicon compound) whereby, depending on the type of process control desired, either only the exposed areas (negative resist) or only the unexposed areas (positive resist) react with the reagent. The non-silylated areas are then dry-developed by etching in an oxygen plasma.
To produce resist structures under 0.5 .mu.m by means of dry-developable single-layer systems, the following resists/processing techniques are generally utilized:
(1) In European Published Patent Application 0 204 253, after exposure, a resist consisting of a novolak resin and a photoactive component is treated at 65.degree. C. with a solution of hexamethylcyclotrisilazane in o-xylol and subsequently dry-developed; both negative and positive-resist structuring are possible.
(2) In European Published Patent Application 0 136 130, a resist based on acrylate is treated with diborane (B.sub.2 H.sub.6) at 40.degree. C. and dry-developed, after exposure to electron beams. A resolution of 0.5 .mu.m is obtained (residual resist thickness: 0.18 .mu.m). Similar results are obtained with a commercially available resist using silicon tetrachloride (SiCl.sub.4) as a treating reagent. Depending on the process control, it is possible to produce positive or negative structures.
(3) In European Published Patent Applications 0 229 917 and 0 251 241, by treating exposed resists with isocyanates and silicon-containing reagents and subsequently subjecting them to dry development, positive images are obtained with suitable process control.
(4) In European Published Patent Application 0 248 779; Proc. of SPIE, Vol. 1086 (1989), pp. 220-228, and J. Vac. Sci. Technol. B, Vol. 7 (1989), pp. 1782-1786, by silylating resists using gas-phase silicon-containing reagents such as hexamethyldisilazane, positive resist images are produced.
The above systems, however, have the following disadvantages: (1) use of corrosive or poisonous, moisture-sensitive gases or liquids; (2) requirement for special apparatus with evacuating capability; (3) treatment at elevated temperatures; (4) long process times; and (5) low O.sub.2 /RIE resistance.
European Published Patent Application 0 394 740 proposes a negative working, dry-developable resist system which, contrary to comparable systems, permits easy handling, features a high selectivity in oxygen plasma, leads to highly resolved structures with steep edges, and finds application using existing equipment. An exceptional feature of this system is that the resist is treated with a metal-containing organic reagent in an aqueous or water-containing, non-toxic phase. This treatment occurs under normal conditions, i.e. at room temperature and standard pressure.
While negative working, dry-developable single-layer systems are generally easier to process than corresponding positive working systems, the positive systems are better suited for applications involving critical contact-hole planes. This, however, is a very important advantage.